Holes in PCBs/PWBs provide two primary functions. Firstly, they accommodate mounting pins for electronic components, and secondly, they provide routing paths between circuit on multi-layer printed circuit boards. Typically these holes can generally be referred to as through-holes and blind holes(or in the case of PWBs, Vias and blind vias). A layer of copper is deposited within the holes by electroplating. There are a number of well-establish techniques within the PCB manufacturing industry for copper electroplating the holes in PCBs. These typically involve a large number of chemical processes before the final electroplating process and such are well within the acknowledge of the skilled addressee.
Known electroplating techniques employ both direct current (DC) and a more recently periodic pulse reverse currents. Although the latter method can eliminate some of the plating defects such as such as low throwing power (the ratio of plating thickness/distribution on the PCB surface to the plating thickness/distribution within the holes), poor distribution across the PCB surface, poor levelling, plating nodules (resulting in a rough finished surface), poor uniformity of copper grain structure and inability to electro-deposit on an even deposit in the holes, some of these defects still occur in plating vias in the micro range.
One particular problem associated with known electroplating processes is the tendency of electrical charges to build up on any imperfections or high-spots on the object/surface being plated. This increased electrical charge attracts more copper than the surrounding areas, which results in a poor finish to the surface of the copper.
In PCB manufacturer it is desirable to use a thick dielectric layer, and thus a thick PCB board, to ensure better functional and electrical performance. This results in high aspect ratio through-holes. A further problem with known electroplating techniques is achieving good plating distribution and a defect-free deposition within the high aspect ratio holes. Holes also represent an “imperfection” in the PCB surface and are prone to a build-up of copper as mentioned earlier.